Some polar molecules lose their Hd+ atoms completely when they dissolve in water. The Hd+ atom then becomes a free H+ ion [2]. Water molecules themselves are constantly dissociating to produces H+ and OH- ions and the ions in turn are re-associating to produce water molecules:

Obviously when water molecules dissociate they produce equal numbers of H+ and OH- ions. Crudely speaking an acid is a substance that only produces H+ ions when it is dissolved in water, thus forming a solution with more H+ ions than OH- ions. For example hydrochloric acid (HCl) dissolves in water to give:

The acidity of a solution is measured using the pH scale which reflects the consecration of H+ ions. A strongly acidic solution has a pH of 1 or 2, a neutral solution has a pH of 7 and a basic (or alkali) solution has a pH between 7 and 14. Bases form OH- ions when dissolved in water (or put another way cause a deficit of H+ ions).

HCl is a strong acid because each molecule will produce an H+ ion. Alcohols on the other hand are very slightly acidic and when a volume of ethanol is dissolved in water a very small proportion of the molecules dissociate:

Organic carboxylic acids, for example ethanoic acid (CH3COOH, found in vinegar), have a COOH group (see Figure 1.4). When ethanoic acid disassociates in water it produces H+ and CH3COO- ions.

Figure 1.4: The ethanoic acid molecule.

Alcohols and carboxylic acids are both found in essential oils, and while carboxylic groups are stronger acids than alcohols they are both relatively weak acids when compared to hydrochloric or sulphuric acids.

Planet Beer Recipes

1.3. Acidity And Dissociation